1. Field of the Invention
The present invention relates to thermal recording devices and methods for manufacture of the printheads used in these devices. More specifically, the invention relates to thermal plotting/printing devices having extended active print width and thus having wide format capabilities. The method of the present invention specifically relates to the manufacture of thermal printheads having extended active print width.
2. Description of the Related Art
Thermal recording devices are devices which use a thermal printhead containing a matrix of small heating elements called dots, nibs or stylli to selectively provide heat to a localized region of a ribbon or recording medium, thereby causing an image to be recorded onto the recording medium. Two types of thermal recording devices are commonly known--thermal transfer devices and direct thermal transfer devices. In thermal transfer devices, a thermal printhead is used to selectively heat an ink-bearing ribbon which transfers the ink to the recording medium (generally ordinary paper). Direct thermal transfer devices use a heat-sensitive recording medium, such as chemically treated paper, which produces the recorded image when selectively heated by a thermal printhead.
In either case, the thermal recording device comprises a thermal printhead and a platen roller with a surface of the platen roller contacting the printhead. The recording medium, and the ribbon in a thermal transfer device, are passed between the printhead and the platen roller so that the recording medium, or the ribbon in a thermal transfer device, is forced against the printhead by the platen roller as the recording medium is moved over the printhead.
A conventional printhead is typically comprised of a substrate base, one surface of which contains a plurality of stylli. The individual stylli may have a dot-type geometry, a serpentine or meandering geometry, or other suitable form. The stylli are positioned on the substrate to form a pattern, typically conforming to a linear matrix arrangement. The printhead includes a bus arrangement which selectively provides electrical energy from an external power supply to the individual stylli in accordance with input signals from a data source. Data source, as that term is used herein, is a device which provides data in the form of electrical signals to be plotted or printed by the thermal recording device. Examples of data sources include microprocessors, process instrumentation, and monitoring devices.
The bus arrangement of the printhead generally includes electrical contacts or leads coupled to the individual stylli. Bus lines are coupled to the leads and run to an external interface circuit which serves a data receiving and switching function. The external interface circuit selectively switches electric current to certain ones of the individual stylli in accordance with the data source input as the recording medium is moved past the printhead, thereby thermally recording the image on the recording medium.
A design for a conventional thermal printhead substrate and method of manufacture are described in U.S. Pat. No. 3,578,946. The stylli are comprised of semiconductor elements on highly resistive substrate base wafers. They are formed by depositing a semiconductor layer on the surface of the substrate base wafer. The semiconductor material is then divided into spaced, parallel, resistive elements by applying a mask and spraying with an abrasive material. Electrical leads are attached to the semiconductor elements by vapor deposition or other conventional means. The wafers thus formed are combined with dielectric wafers in a sandwich-type arrangement to form a printhead substrate. This substrate, when combined in a housing with a switching means such as the external interface circuit discussed above, constitutes a conventional printhead.
The term printhead substrate or substrate as used herein refers to the portion of a thermal printhead including the substrate base, the stylli attached to or integrated into the substrate, the leads, bus structure, and data receiving and switching circuits.
The recording medium is moved between the printhead and the platen roller, and thus over the printhead, in a direction perpendicular to the axis of the platen roller. The width of the device as that term is used herein refers to the linear dimension along the longitudinal axis of the platen roller. The width of the printhead is the width of the substrate surface contacting the ribbon or the recording medium. The active print width is the width along the printhead measured from the first row of active stylli to the last row of active stylli, and thus corresponds to the width of the recording medium for which an image can be directly created by the printhead. Active print depth is the linear dimension of the printhead perpendicular to the print width and parallel to the direction of paper movement for which there are active stylii of about 1.0 mm (0.04 in.).
Thermal recording devices have become increasingly important in recent years as development of the underlying technologies has progressed and new applications have been found. Some of these new applications have created the demand for wide format devices. For example, it is typically necessary in a number of fields to compare the values of several variables relative to the value of one or more independent variables, such as time. This comparison can be greatly facilitated by plotting the data using a strip chart-type format. However, the width of the ordinate axis for each variable plotted must be sufficient to discern small variations in the value of the respective variable. Thus, there is demand for a thermal plotter/printer with wide format capabilities sufficient to accomodate plots of several variables while providing for small variations in the value of each variable. Examples of applications requiring wide format capabilities include seismic analysis of geologic structures, patient monitoring and other medical applications, and monitoring of performance parameters for aircraft, spacecraft, missiles, and remotely piloted vehicles. CAD/CAM applications similarly may require wide format plotting and printing for adequate resolution of detail.
Thermal printheads of conventional design have been limited in width primarily by the fragile character of the substrates. The susceptibility of the substrate to breakage increases with increasing length. This has had the practical effect of limiting the maximum length of such conventional printheads to about 25 cm (10 inches).
It is an object of the present invention to provide a thermal recording device having wide format capabilities. It is further an object of the present invention to provide a thermal printhead with an extended active print width. It is still further an object of the present invention to provide a method of manufacture of a thermal printhead having an extended active print width.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.